LED Resistor Calculator
Find the right resistor value and wattage for driving an LED.
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Every LED needs a current-limiting resistor to prevent burnout. The resistor value depends on the supply voltage, LED forward voltage and desired current. This calculator gives you the exact resistance and power rating needed. Always choose a resistor with at least 2x the calculated power dissipation for reliable operation.
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Frequently asked questions
For a typical red LED (2V forward, 20 mA), R = (5 - 2) / 0.02 = 150 Ω. The resistor dissipates 60 mW so a standard 1/4W resistor works fine.
Check the datasheet. Typical values: red = 1.8-2.2V, green = 2.0-3.0V, blue/white = 3.0-3.4V. Infrared LEDs are around 1.2-1.5V.
Yes, for LEDs in series, add up all forward voltages. For 3 red LEDs at 2V each on a 12V supply: R = (12 - 6) / 0.02 = 300 Ω.
The LED draws excessive current and overheats. At 2x rated current most LEDs fail within hours. At 5x they fail instantly. Always calculate the correct value.
Use at least 2x the calculated power dissipation. A resistor dissipating 60 mW should be rated at least 125 mW (1/8W). Standard 1/4W (250 mW) resistors work for most single-LED circuits.